using System;

    class Program
    {
        static void Main()
        {
            int a = 1;
            int N;
            N = int.Parse(Console.ReadLine());
            if(Math.Abs(N)<10000)
                 if (N >= 1)
                    Console.WriteLine(((a + N) / 2) * N);
                 else
                    Console.WriteLine(((a+Math.Abs(N)) / 2) * N+1);
            else
            Console.WriteLine("ERROR");
          }
}

Edited by author 18.09.2013 00:10

Edited by author 18.09.2013 00:12

what if the output is N=2

(a+N)/2=1.5  1.5*2=3.0 this is not an integer

better: (N+1)*N div 2

if N<0:
   -((N+1)*N div 2) + 1